Hot stack gases which issue from a blast furnace or the like at a pressure of several atmospheres are usually passed through a cleaning or purifying device which first separates out the larger particles carried by the gas and then subjects the gas to a scrubbing operation which removes many of the water-soluble gas components and removes additional particles from the gas stream. The above-cited earlier applications describe so called Venturi washers which comprise a Venturi tube having a narrow waist in which is received a Venturi body that is displaceable within this tube so as to define a variable gap therewith. A sprayer is provided in the upstream end of the tube so that the turbulence and pressure drop in the Venturi will thoroughly wash the remaining particles from the gas.
Such Venturi washers are used to maintain a constant back pressure in the blast furnace. A constant back pressure is necessary for proper functioning of the furnace and can readily be maintained by the Venturis. At the same time such devices serve to drop the pressure so that the cleaned stack gases can be used in regenerators for heating up the charge or the air that goes into the furnace. As a rule a plurality of such Venturi or annular-gap washers are used with all of their Venturi bodies ganged so as to permit adjustment of the pressure drop across them to maintain the pressure in the blast furnace constant.
Such systems are relatively effective. However they have the principal disadvantage that they waste a considerable amount of energy present in the hot pressurized stack gas. A significant amount of energy available to do work is wasted as the gases pass through the Venturis and expand.